1. Field of the Invention
The present invention relates to an automatic transmission which transmits a motive power of a prime mover to driven parts by switching between one and the other of two power transmission paths, and executes pre-shift control when executing switching between the power transmission paths.
2. Description of the Related Art
Conventionally, an automatic transmission for a vehicle is known as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2008-180320. This automatic transmission transmits motive power of an engine to wheels while changing the speed of the motive power, by switching between one and the other of two power transmission paths, and also executes pre-shift control when executing switching between the power transmission paths. The automatic transmission is a five-speed transmission, and includes a first input shaft and a second input shaft which are connected to the engine via a first clutch and a second clutch, respectively, an output shaft connected to the wheels, first to fifth driven gears relatively rotatably provided on the output shaft, three synchronization mechanisms which connect the driven gears to the output shaft while synchronizing the driven gears with the output shaft, three drive gears concentrically fixed to the first input shaft and in constant mesh with three odd-numbered driven gears, and two drive gears concentrically fixed to the second input shaft and in constant mesh with two even-numbered driven gears.
The automatic transmission executes pre-shift control, as shown in FIG. 3 in Japanese Laid-Open Patent Publication (Kokai) No. 2008-180320. In the pre-shift control, in a state in which the motive power of the engine is transmitted to the wheels via one of the two power transmission paths, while holding a clutch for the other power transmission path in a disengaged state, one of the synchronization mechanisms, which is associated with the other power transmission path, connects beforehand between one of the driven gears forming the other power transmission path and the output shaft. As shown in FIG. 3 in Japanese Laid-Open Patent Publication (Kokai) No. 2008-180320, a target speed position is set in a step 301, and in a step 302, the associated synchronization mechanism is driven to connect a driven gear for the set target speed position to the output shaft while synchronizing the same with the output shaft. Then, in a step 303, it is determined whether or not the connection of the driven gear for the target speed position to the output shaft is successful. If the connection of the driven gear for the target speed position to the output shaft has failed, the connection operation by the synchronization mechanism is cancelled in a step 304, and engagement control of a clutch between an input shaft associated with the other power transmission path and the engine is performed in a step 305, followed by terminating the pre-shift control.
Further, the present applicant has proposed a synchronization mechanism in the Publication of Japanese Patent No. 4242189. This synchronization mechanism connects a transmission gear to a rotating shaft while synchronizing the transmission gear with the rotating shaft, and includes an actuator, a sleeve, a blocking ring, a synchronization spring, and so forth. In this synchronization mechanism, when the sleeve is driven by the actuator, spline teeth of the sleeve press the blocking ring toward the transmission gear via the synchronization spring. This generates frictional forces between the blocking ring and the transmission gear, to rotate an outer ring of the blocking ring, whereby the spline teeth of the sleeve are brought into contact with ring gear teeth of the blocking ring. As a consequence, the sleeve is synchronized with the blocking ring, and then the spline teeth of the sleeve are in mesh with dog teeth, whereby the transmission gear is connected to the rotating shaft.
In general, in the case of an automotive vehicle using an engine as a motive power source, when engine torque is rapidly increased by a stepping-on operation of the accelerator pedal by a driver, a reaction force is generated by frictional resistance between wheels of the vehicle and a road surface. This reaction force makes torque fluctuation liable to occur on the output shaft, since the output shaft is connected to the wheels. When torque fluctuation occurs on the output shaft as described above, according to the above-described conventional automatic transmission, since one output shaft is shared by the two power transmission paths, one of the synchronization mechanism executing the pre-shift control sometimes fails in connecting the driven gear for the target speed position to the output shaft or suffers from temporary failure of synchronization.
When the connection operation by the synchronization mechanism has failed as described above, the connection operation has to be executed again after engagement of the clutch. This can cause an increase in the number of times of execution of the connection operation, causing shortening of the service life of the synchronization mechanism. Further, when such a synchronization mechanism as proposed in the Publication of Japanese Patent No. 4242189 is used as the synchronization mechanism described above, if the synchronization mechanism suffers from temporary failure of synchronization, the spline teeth of the synchronization sleeve and the ring gear teeth of the blocking ring can hit against each other to produce gear noise. In this case, the service life of the synchronization mechanism is shortened and marketability is degraded.